Chemical mechanical polishing (CMP) is one method of providing a planarized substrate surface. Such substrates may be used in the manufacture of integrated circuit devices, for example. CMP may be used to planarize raw substrates, to completely or partially remove a bulk deposited layer, or to planarize a surface by partially removing layers which have been deposited over a non-planar feature formed on a substrate. For each of the above operations, it is critical to detect when a desired amount of material has been removed. Once this "endpoint" condition has been attained, the polishing operation may be terminated, or it may be adjusted to proceed to another polishing operation having different polishing characteristics.
A typical CMP apparatus employs a rotating polishing surface, such as a consumable polishing pad, against which the surface of the substrate being polished, is placed. The CMP apparatus also includes a carrier which secures the substrate in a desired position with respect to the pad. The carrier includes means for providing a force to keep the substrate in contact with the pad, and also may include means for rotating, vibrating, or oscillating the substrate. Most commonly, the carrier is centrally connected to a spindle which rotates the carrier and substrate being polished, and provides the force which urges the substrate being polished, towards the polishing pad. During polishing, a slurry having both chemical and abrasive agents may be supplied to the interface between the substrate and the pad to enhance the rate at which material is removed from the substrate.
One problem associated with CMP is endpoint detection. Endpoint may be defined as the point at which the desired polishing process is completed. When "endpoint" is attained, a number of different actions may be taken in response. For example, the entire polishing process may be terminated when endpoint is attained or the polishing conditions may be changed as the polishing process continues, with another polishing operation, to polish an underlying material. For example, the process conditions may be changed to produce a final buffing operation, or they may be changed to produce another polishing operation having different characteristics to polish an underlying film. It can be seen that a substrate containing a stack of films to be polished, may include a number of discrete polishing operations, each of which includes an associated "endpoint."
Depending on the chemical mechanical polishing operation being performed, "endpoint" may signify different events. For example, when polishing a raw substrate, "endpoint" may be achieved when a certain predetermined substrate thickness has been removed. The same is true for a layer or film which is being partially removed. When a film is being completely removed from a substrate, "endpoint" is attained upon complete removal of the film. When CMP is used to planarize a substrate by removing portions of a film which extend above underlying features, "endpoint" is achieved when the surface is essentially planar. Generally speaking, "endpoint" is achieved after a predictable amount of material is removed from the surface. It is therefore necessary to accurately detect when a prescribed amount of material has been removed from the surface being polished. Once the prescribed material thickness has been removed, the polishing operation may be quickly terminated or otherwise adjusted, at that point. Because the substrate is polished face-down and the polishing surface is generally contiguous with the polishing pad, a process monitor cannot easily be used to continuously monitor the surface being polished. As such, it is difficult to attempt to use such a monitor to determine the polishing "endpoint." Optical or spectral means are generally not available as endpoint detectors which continuously monitor the surface being polished.
Variations in the polishing conditions also impede an accurate determination of the polishing endpoint. For example, variations in the slurry flow is rate and composition, pad condition, relative speed between the pad and the substrate, the material being polished, and the load of the substrate on the pad, may cause variations in the material removal rate. These variations in the material removal rate cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot reliably be estimated merely as a function of polishing time.
One approach to predicting the polishing endpoint is to periodically remove the substrate from the polishing surface and measure the thickness of the substrate or the film being removed by polishing. By periodically removing the substrate from the polishing surface and measuring its thickness, the rate of material being removed from the substrate may be determined. As such, a linear approximation of the material removal rate may be used to determine the polishing endpoint. This method is time consuming, requires repeatedly removing the substrate from the polishing operation, and does not account for sudden changes in the removal rate that may occur between measurement intervals. Moreover, this method does not account for the other variations in the material removal rate, as discussed above.
Several other non-invasive methods of endpoint detection are known. These methods generally fall into two categories: those which require access to the surface of the substrate being polished, and those which determine the polishing endpoint by determining changes in the operation of the polishing apparatus.
Methods included within the first category typically require real-time access to at least a portion of the substrate surface being polished, such as by sliding a portion of the substrate over the edge of the polishing pad and simultaneously analyzing the exposed portion of the substrate. For example, where polishing is used to remove the bulk of a conductive film such as a metal, and to expose a subjacent dielectric layer, the overall or composite reflectivity of the surface being polished changes as the bulk metal film is removed and the dielectric layer is exposed. By monitoring the reflectivity of the polished surface or the wavelength of light reflected from the surface, the polishing endpoint can be detected as the reflectivity changes when the dielectric layer is exposed. However, this method is limited to determining the polishing endpoint when a first film is being completely removed from over a second material which has a reflectivity which varies from the first film being polished. Additionally, it is somewhat erratic in predicting the polishing endpoint unless all of the underlying surface of a different reflectivity, is simultaneously exposed. Furthermore, the detection apparatus is delicate and subject to frequent breakdown caused by the exposure of the measuring or detecting apparatus to the slurry.
Methods for determining the polishing endpoint included within the second category, do so by monitoring various parameters of the polishing apparatus and indicating that endpoint has been attained when one or more of the parameters abruptly changes. An example of such a parameter is the coefficient of friction at the interface of the polishing and the substrate. When a metal layer is being polished to expose an underlying dielectric layer, the coefficient of friction will change when the dielectric layer is exposed. As the coefficient of friction changes, the torque necessary to provide the desired polishing pad speed also changes. By monitoring this change such as by monitoring the polishing motor current, endpoint may be detected. However, the coefficient of friction is a function of the slurry composition, the pad condition, the load of the substrate on the pad, and the surface condition of the substrate, and, as the substrate is being polished, the pad condition and the slurry composition at the pad-substrate interface, change. Moreover, vibration and electrical noise may distort the characteristic being measured. Such effects may mask the change in the characteristic being monitored and may endpoint the polishing operation at the incorrect time. Additionally, this method of endpoint detection will only work if the polishing operation is used to expose an underlying material having a frictional attribute different than that of the material being removed.
Another method for determining endpoint included within the second category involves monitoring the current supplied to each of the polishing motors, and determining that endpoint has been achieved when a predetermined amperage sum has been reached. Like the other methods included within the second category, this method can also be adversely influenced by the above effects.
It can be seen that none of the available endpointing methods described above, detect endpoint by directly monitoring the amount of material being removed from the surface being polished, without interrupting the polishing process. As such, none of the known methods for determining endpoint, do so by measuring the amount of material being removed, or by measuring the corresponding thickness loss of the substrate being polished, during the actual polishing operation. It can be understood, then, that such a method is desirable in the art of CMP.
It can be further seen that there is a need to provide a method for detecting endpoint when a prescribed amount of material has been removed from a substrate in order to terminate or otherwise adjust the polishing process at this point.